British police raided several properties and arrested seven people in connection with the attack outside Parliament that left four dead, including the man who mowed down pedestrians on a bridge and fatally stabbed an officer, a senior police official said.

Metropolitan Police counterterrorism chief Mark Rowley said that he believed the attacker acted alone and was "inspired by international terrorism."

Police raided six addresses, including some in the central city of Birmingham, and arrested seven people in connection with Wednesday's attack by a knife-wielding man, Rowley said.

Rowley refused to identify the attacker, who was killed by police after fatally stabbing a police officer on Parliament's grounds after striking pedestrians with a vehicle on Westminster Bridge.

The attacker has been identified and was known to British security, according to a British security official who spoke on condition of anonymity because he was not authorized to speak about ongoing security operations. He declined to name the man and to give any other details about his identity, nationality or hometown.

Rowley revised the death toll from five to four, including the attacker, the police officer and two civilians. He said that 29 people required hospitalization and seven of them are in critical condition. He also said that authorities were still working out the number of "walking wounded." Police had previously given the total number of injured as around 40.

Rowley said investigation were continuing around Parliament but expected that lawmakers would be able to go ahead with plans to reconvene in a show of solidarity.

Before Rowley's news conference, British media reported that armed police carried out a raid on a property in Birmingham. The Press Association on Thursday quoted an unnamed witness saying that the operation was linked to the attack. The witness said that police raided an apartment and arrested three men. Police in the West Midlands, where Birmingham is located, directed inquiries about the operation to London's Metropolitan Police.

The attacker drove an SUV into pedestrians on Westminster Bridge before crashing the vehicle into the gates of Parliament on Wednesday. He scaled the fences and later fatally stabbed a policeman before being gunned down by officers.

On Wednesday, Prime Minister Theresa May condemned the rampage as a "sick and depraved terrorist attack."

Lawmakers, lords, staff and visitors were locked down after the man was shot by police within the perimeter of Parliament, just yards from entrances to the building itself and in the shadow of the iconic Big Ben clock tower.

A doctor who treated the wounded from the bridge said some had "catastrophic" injuries. Three police officers, several French teenagers on a school trip, two Romanian tourists and five South Korean visitors were among the injured.

The threat level for international terrorism in the U.K. was already listed at severe, meaning an attack was "highly likely."

Speaking outside 10 Downing St. after chairing a meeting of government's emergency committee, COBRA, May said Wednesday that level would not change. She said attempts to defeat British values of democracy and freedom through terrorism would fail.

Londoners and visitors "will all move forward together, never giving in to terror and never allowing the voices of hate and evil to drive us apart," May said.

President Donald Trump was among world leaders offering condolences.

London has been a target for terrorism many times over past decades. Just this weekend, hundreds of armed police took part in an exercise simulating a "marauding" terrorist attack on the River Thames.